feat(hud): render the Intel HUD from a store

IntelHUD no longer builds #intel-hud with innerHTML or writes its readouts
by id: it publishes its readouts, typing summary, REC blink, visibility,
layout variant and style colors through its publish option to the gev-hud
store, and the intel-hud application view renders them. The corners render
from mount so the panel layout controllers register them as obstacles.

The vessel layer presents the selected vessel's readout through
src/hudReadouts.js, which IntelHUD follows, instead of writing into the
HUD. The Cyber sonar state answers a HUD toggle from IntelHUD's visibility
rather than the class the view renders on its next update.

src/hudReadouts.js

@@ -0,0 +1,29 @@
+/**
+ * Readouts other owners show in the Intel HUD. The vessel layer presents the
+ * selected vessel here and the HUD subscribes, so neither reaches into the
+ * other's markup.
+ */
+
+/** @type {?{text: string}} The selected vessel's lines, or null for none. */
+let selectedVessel = null;
+const vesselListeners = new Set();
+
+/**
+ * Show the selected vessel's readout, or clear it.
+ * @param {?{text: string}} readout Its lines joined by newlines, or null.
+ */
+export function presentSelectedVessel(readout) {
+  selectedVessel = readout ? Object.freeze({ text: readout.text }) : null;
+  for (const listener of vesselListeners) listener(selectedVessel);
+}
+
+/**
+ * Follow the selected vessel's readout; the current one is replayed at once.
+ * @param {(readout: ?{text: string}) => void} listener
+ * @returns {() => void} Stop following.
+ */
+export function subscribeSelectedVessel(listener) {
+  vesselListeners.add(listener);
+  listener(selectedVessel);
+  return () => vesselListeners.delete(listener);
+}